Transferring biologically active molecules, such as, for example, DNAs, RNAs or proteins, into living cells is an important tool for analysis of biological functions of these molecules. Electroporation is a preferred method for transferring foreign molecules into the cells, which in contrast to chemical methods causes less undesirable changes of the biological structure and function of the target cell. During electroporation the foreign molecules are introduced into the cells from an aqueous solution, preferably a buffer solution adapted to the cells or a cell culture medium, by a short-time current flow, i.e. the pulse of a discharging capacitor, whereby the cell membrane is made permeable for the foreign molecules by effect of the short electric pulses. Solution and cell suspension, respectively, are usually provided in a so-called cuvette, i.e. a small container which is open at the top and which comprises two oppositely and parallel arranged electrodes disposed in the sidewalls near the bottom and serving for the application of electric voltage. Through the temporarily emerging “pores” in the cell membrane the biologically active molecules initially reach the cytoplasm where they eventually already exert their function to be analysed. At certain conditions the molecules subsequently also enter the nucleus of the cell. Particularly with the introduction of DNA into animal cells, the so-called transfection, specific problems often arise during electroporation because of the fragility of the cells since the efficiency of transfection is affected by the survival rate of the cells as an important parameter.
Due to the temporarily applied intensive electric field, i.e. a short pulse with high current density, cells, derivatives of cells, subcellular particles and/or vesicles can also be fused. During this so-called electrofusion at first, for instance, the membranes of the cells are brought in close contact by an inhomogeneous alternating electric field. The subsequent application of an electric field pulse leads to an interaction of parts of the membranes, which finally leads to cell fusion. Comparable devices such as the ones used for electroporation can be used for electrofusion as well.
Containers as mentioned above are known and primarily used for electroporation or electrofusion in the form of cuvettes having inserted electrodes made of metal. Containers used for this purpose are mostly small vessels which are closed at the bottom and open at the top and whose inner space is build by two pairs of parallel and oppositely arranged sidewalls. The inner space serves for receiving of the cell suspension, i.e. usually an aqueous buffer solution or a cell culture medium, in which the cells to be treated are suspended. Such cuvettes mostly comprise a pair of electrodes for application of an electric voltage disposed near the bottom of a pair of oppositely arranged sidewalls. During an electric discharge an electric current flows through the cell suspension between both electrodes, which enables an introduction of nucleic acids or other molecules into the cells or, depending on the selected conditions, leads to fusion of cells. The electrodes are mostly made of metal, wherein aluminium is frequently used. But it is an disadvantage of these known, commercially available cuvettes that metal ions are emitted into the buffer solution during the electric discharge, which can cause an undesirable stimulation of the cells at lower concentrations or, at higher concentrations, act toxic on the cells. For instance, using cuvettes made of aluminium a negative effect due to the release of Al3+ ions could be demonstrated (Loomis-Husselbee et al., Biochem J 1991, 277 (Pt 3), 883-885). Furthermore, using cuvettes having electrodes made of metal generation of undesirable precipitates may occur, which are also generated due to the release of metal ions from the electrodes. The precipitates may be metal hydroxides or complexes of metal ions with biological macromolecules of the buffer solution (Stapulionis, Bioelectrochem Bioenerg 1999, 48(1), 249-254). Finally, it is another disadvantage of cuvettes made of aluminium that the resistance of the cuvettes decreases during discharge, presumably because a layer of oxidized aluminium having a higher resistance is released from the electrode by the current flow. Additionally, cuvettes having electrodes made of metal are difficult to produce and very expensive.
U.S. Pat. No. 6,001,617 discloses a device for cultivation of cells, which can be used for electroporation as well as for electrofusion of cells. The device consists of a round container having an optically transparent bottom on which a layer of cells can adhere and grow. The bottom of the container may consist of an optically transparent, non-conductive material being coated with an electrically conductive material, or can be completely made of an optically transparent and electroconductive material. The electroconductive bottom is contacted via a band-like electrode made of metal and being circumferentially disposed in the wall area. To act as counter electrode a likewise circular band is provided. It is one drawback of this known device that it is primarily provided and suitable for electroporation of adhering cells. Using this device for transfection of suspended cells is limited and merely possible if low efficiency is accepted. Due to the ring-like circular contacting of the bottom electrode and the counter electrode being likewise circumferentially disposed in the wall area a homogenous electric field cannot be generated so that equal transfection of all cells cannot be achieved. This effect is increased by the fact that the intrinsically conductive plastics used have a higher resistance as they are provided as thin coating. Thus, the cells adhering to the bottom in the centre of the surface can only be transfected with very low efficiency. Furthermore, because of the sophisticated construction and the fact that the intrinsically conductive plastics used are not mouldable, the production of the known container is very expensive.